1. Field of the Invention
The present invention relates in general to a superconducting microwave device, and more particularly, to a superconducting tunable filter applied to an ultralow temperature RF front-end of a transmitter of a base station in mobile communications systems.
2. Description of the Related Art
Along with rapid development and spread of mobile phones in recent years, high-speed and high-volume data transmission technologies have becomes indispensable. Because of extremely small surface resistances, as compared with typical good conductors, superconductors have great potential for application to RF filters used in base stations of mobile communications systems, and application to low-loss and high-Q resonators are especially expected.
As illustrated in FIG. 1, an RF signal received at an antenna (ANT) 151 is processed through a bandpass filter (BPF) 152R, a low-noise amplifier (LNA) 153, down converter (D/C) 154, and a demodulator (DEMOD) 155, which components constitute a receiver RF front-end, and then processed at a baseband processing unit 156.
At the transmitter RF front-end, a baseband-processed transmission signal is subjected to processes successively through a modulator (MOD) 157, an up converter (U/C) 158, a high power amplifier (HPA) 159, and a band-pass filter (BPF) 152T, and then transmitted as an RF signal from the antenna 151.
When a superconducting filter is applied to the receiving-end bandpass filter 152R, transmission loss is small and a steep frequency cut-off characteristic can be expected. When it is applied to the transmission-end bandpass filter 152T, an effect of removing distortion generated due to the high power amplifier 159 can be expected. However, the transmitter RF front-end needs a high power system to transmit radio frequency signals, and therefore, today's issue is balancing the compact structure and satisfactory power quality.
In application to the mobile communications field, frequency tunability is in strong demand. In order to achieve a tunable superconducting filter, it is proposed to arrange a plate having a surface covered with a conductive film above a pattern of a superconducting resonator such that the conducting surface faces the superconducting resonator. A piezoelectric device is inserted between the superconducting resonator and the conducting face of the plate to adjust the distance between the two to vary the resonant frequency. See, for example, WO 01/041251.
Because this method uses a mechanical mechanism of an actuator, there are problems of high susceptibility to shaking or vibration and slow response speed.
Another known technique for achieving frequency tunability is to make use of a dielectric having a highly bias-dependent permittivity. It is proposed to form a dielectric film with a bias-dependent permittivity over the pattern of a resonator filter, and to apply an electric voltage to the dielectric film to vary the dielectric constant. See, for example, JP 9-307307A. In this method, an electric voltage is applied in the lateral direction, and the rate of change is small. In addition, since the power durability of this filter device is insufficient, it is only applicable to the receiving front-end.
Still another known method is to place a superconducting dielectric resonator of a parallel plate type on a microstrip line and tune the frequency characteristic of the resonator by making use of the bias-voltage dependency of the permittivity of the dielectric plate. See, for example, WO 97/23012.
FIG. 2A and FIG. 2B illustrate the structure of a tunable superconducting filter disclosed in WO 97/23012. In FIG. 2A, the resonator 111 has a dielectric plate 112 with a nonlinear permittivity, and high temperature superconductor (HTS) plates 113a and 113b are arranged on both sides of the dielectric plate 112. The HTS plates 113a and 113b are covered with conducting films 114. FIG. 2B depicts the resonator 111 (FIG. 2A disposed on a microstrip line 115. One of the superconducting plates (113b in this example) is electrically connected to the center strip 118 of the microstrip line 115 via the conducting film 114, as illustrated in FIG. 2B. A signal propagates through the resonator 111 from a particular side (designated as INPUT) to the side (designated as OUTPUT) opposing to the particular side. A DC bias voltage is applied between the upper superconducting plate 113a of the resonator 111 and the microstrip line 115 using a voltage supply 119. By changing the DC bias voltage, the dielectric constant of the nonlinear dielectric plate 112 is changed to vary the resonant frequency of the resonator 111.
This structure, however, is inferior in power durability, and therefore, it is applicable only to a filter of a receiving-end. Poor power durability in the conventional superconducting filter is attributed to concentration of electric current at the corners or edges of the superconducting resonator patterns.
It may be effective to form a resonator in a patch pattern or a plane FIGURE pattern, including a disk pattern, an oval pattern, an elliptic pattern, and a polygonal pattern, with less sharp corners or edges. Such shapes are effective in reducing local concentration of electric current on the superconducting resonator, and a large power response required for a transmission filter can be achieved. Such a patch shaped (plane FIGURE shaped) superconducting filter may be further developed by arranging a conductive pattern with a certain shape above the superconducting resonator via a dielectric between them to cause coupling corresponding to a desired bandwidth. By generating two orthogonal resonating modes (dual mode) in a round or polygonal resonator, the power characteristic and the frequency characteristic can be improved through reduction of concentration of electric current, and the device can be made compact because of the dual-mode structure.
However, the above-described dual-mode resonator does not have frequency tunability, and it cannot deal with correction of deviation in characteristic features due to variation in manufacturing nor with positive adjustment of characteristic features.